PPCs are no longer a central concept to our model, but instead a
feature that that pulumi.com provides to some organizations. Let's
remove most mentions of PPCs except for cases where we really need to
talk about them (e.g. when a stack is actually hosted in a PPC instead
of just via the normal pulumi.com service)
Also remove some "in the Pulumi Cloud" messages from the CLI, as using
the Pulumi Cloud is now the only real way to use pulumi.
Fixespulumi/pulumi-service#1117